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Blood Clots & Travel: World Health Organization (WHO) Report on Travel and Clotting Risk

Henry I. Bussey, Pharm.D., FCCP, FAHA
July, 2007

In 2001 the World Health Organization (WHO) embarked on a two-phase project called the WHO Research Into Global Hazards of Travel (WRIGHT) Project. The first phase was to confirm that the risk of venous thromboembolism (VTE - blood clots in the veins) is increased by air travel, to assess the size of the risk, and to evaluate the impact of other factors. The second phase will examine the value of preventive measures and has not yet been completed.

The findings of the WRIGHT Phase I report are largely consistent with studies previously highlighted on ClotCare (

Key findings include:

  1. Although individual studies have provided conflicting results as to whether prolonged air travel increases the risk of VTE, the WRIGHT report concluded that the risk of VTE roughly doubles with flights greater than 4 hrs and increases with longer flights or repeated flights within a short period of time.

  2. Overall, the absolute risk of VTE appears to be about 1 in 6,000 healthy individuals, but the risk is primarily concentrated in those with hypercoagulable conditions, those using birth control pills, and/or those with other risk factors such as obesity, extremes of height (less than 5'2" or greater than 6'2"), older age, and cardiovascular disease. Note that a hypercoagulable condition is a biochemical abnormality that increases clotting risk.

  3. The VTE risk is greatest immediately following the flight but some risk persists for several weeks afterwards.

  4. The risk also exists for other forms of travel, but travel-related immobility may carry a greater risk than non-travel immobility (such as sitting in a movie marathon).

Phase II of the WRIGHT report will address the merits of various preventive measures for travel related VTE.

Below are some links that you may want to review for additional information.

WHO Press release:

WHO report:

Associated press story:

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